Humans may have acquired certain immune system genes by breeding with Neanderthals. And while these genes may have helped them avoid diseases, they might be the cause of others.

According to the Guardian, scientists have long known that early humans got it on with related species, the Neanderthals and Denisovans: "The Neanderthals contributed up to 4% of modern Eurasian genomes, while the Denisovans contributed roughly 4-6% of modern Melanesian genomes. That doesn't happen by holding hands." Now a new study shows that among the contributed genes were some called HLA class I, which affect the immune system. This probably had good effects, giving humans some of the same immunities as the other species, which were better adapted to their environment. However, it may also have caused problems — the genes are linked to autoimmune disorders. Says study co-author Paul Norman,

The vast majority of autoimmune diseases have been shown by genome-wide association studies to be associated with particular HLA alleles and we find a couple of those in Denisovans. So it looks to me like modern humans have acquired these alleles, but we weren't kind of prepared for them, we hadn't grown up with them, and in some circumstances, they can start to attack us as well as the viruses and other pathogens.

HLA aren't the only genes that can cause some diseases and prevent others — the same gene associated with sickle cell anemia also confers resistance against malaria. And it's also true that genes that were protective or neutral when humans didn't live very long can turn dangerous now that many of us survive into old age. Researchers are still studying the HLA genes, and it's not clear if this new finding will lead to better treatments for autoimmune disorders. But at the very least, it's a reminder that early humans weren't alone — and that we carry the marks of our fellow species to this day.